Employee was loosening nuts on the trunion of a ball mill. He was using a 16 lb. sledgehammer and 2.5 in. slug wrench. The hardware was very tight and he had hit one of the nuts several times with the sledgehammer. The last time he hit that nut, it loosened and he lost his footing. His body flew into the mill and he felt shoulder pain. That evening the pain worsened.